This invention relates to a digital communication system and, more particularly, to a digital communications system for processing digital characters to mitigate overload.
In a pulse code modulation (PCM) system, a typical analog-to-digital transmitter encoder functions to quantize an analog message sample and to generate a digital character. In nonuniform PCM encoding, for example, encoding according to a .mu. = 255 companding law such as is utilized in the T-Carrier System of the Bell System, each analog sample is converted into an eight-bit digital character. The initial bit of the digital character represents the sign or polarity of the analog sample and is hereinafter labeled S. The coarse amplitude of the analog sample is represented by the three bits following the sign bit and is labeled ABC hereafter. Each combination of the ABC bits, known as a segment value, represents one of eight amplitude ranges. The value of consecutive segments corresponds to a doubling in magnitude of the analog sample. Finally, the remaining four bits, called the mantissa bits and here labeled WXYZ, represent one of sixteen usually equal length intervals present in each one of the segments. Once generated, each digital character "SABCWXYZ" is transmitted over a communication link to a receiver. For example, in the Bell System T-Carrier System, a character is multiplexed into a time slot of a 193-bit frame and the frame then transmitted at a nominal bit rate of 1.544 megabits per second (Mb/s) over a digital communication link to the receiver. The frame usually includes a framing signal and a plurality of characters, each character from a different one of a plurality of input trunks, typically 24 trunks. At the receiver, the frame is advantageously demultiplexed so that each character can be extended to a respective output trunk.
In a large telecommunication system, it is economically desirable to efficiently utilize the communication link as, for example, by reducing bandwidth. As a result, various transmitter bandwidth reduction arrangements exist in the prior art for compressing a large number of trunks, e.g., N trunks, onto a communication link having a smaller number of channels, e.g., C channels, (C&lt;N), with a typical compression ratio N/C of about three-to-one. Of course, the receiver usually includes an inverse arrangement for expanding each channel to the respective trunk. On the one hand, no actual bandwidth reduction is obtained if the number of trunks having active speech as detected by a speed detector does not exceed the number of channels, since each active trunk is then assignable to a channel. On the other hand, if the number of active trunks, labeled N.sub.A, exceeds the number of channels, the exceeding being called an overload, a substantial loss of information may obtain. As one would expect, the loss gives use to a signal degradation.
Overload and its unfortuitous effects can be mitigated in a digital speech interpolation (DSI) system by increasing the effective number of channels provided on the link. Of course, as the effective number of channels becomes equal to the number of active trunks, N.sub.A, overload is obviated. However, its unfortuitous effects including insertion of additional quantization noise in the active speech signal usually remain. In known DSI systems an increase in the effective number of channels is often achieved using apparatus for partitioning a fixed-length PCM output frame into a data field of D bits and an activity status field of A bits.
Within the data field, digital characters are advantageously formatted so that, for an eight-bit character, a typical data field bit length is eight times the number of channels, i.e., D = 8C bits. It is clear, therefore, that, in a fixed length data field, the effective number of channels on the link can be increased by reducing the number of bits employed to encode the analog sample. For example, by compressing each digital character from eight bits to six bits per character, the number of effective channels increases from D/8 to D/6 channels, an increase in excess of 33 percent. Hence, bandwidth is reduced and overload is mitigated.
In one compression arrangement having a character from each active trunk, it is common that each character in the data field of length D bits be compressed so that all N.sub.A compressed characters have the same number of bits, e.g., Q = D/N.sub.A bits per character. Illustratively, for Q = 6 the character "SABCWXYZ" could be compressed to "SABCWX" with the least significant "YZ" bits being truncated. However, inasmuch as D/N.sub.A may not be an integer, that arrangement often results in unused bits in the data field. In another compression arrangement, the compressed characters in the data field differ by at most one bit, i.e., the length of some characters is Q bits while that of others is (Q + 1) bits. Although in the second arrangement all bits in the data field can be used, both compression arrangements increase quantization noise. Also, both arrangements typically employ an arithmetic unit to precalculate the number of bits per compressed character. However, as between the two, the second typically results in less quantization noise than the first and, accordingly, has heretofore been a preferable arrangement for mitigating overload and its unfortuitous effects.
Of course, signaling arrangements are employed between transmitter and receiver for the former to notify the latter of a correspondence between trunk and channel. Such is a function of the aforementioned activity status field. In the status field, there are usually one or more bits assigned to each trunk. For example, in a status field having one bit allocated to each trunk, i.e., A = N, a logic one in bit i could mean that trunk number i is active; whereas a logic zero could means the trunk is inactive. Thus the number of logic ones in the status field could equal the number of active trunks on the communications link. It is known that still further mitigation of overload is possible by efficiently processing the activity bits. For example, if the activity field in each frame is arranged to signal the activity of a lesser number than N trunks, the length of the data field can be increased. In particular, if the frame bit length remains a constant, it is clear that, as the number of bits allocated in the partitioning to the status field is decreased, the bit length of the data field can be increased. Further, as the length of the data field D is increased and the number of bits per digital character Q remains constant, or even decreases, the effective number of channels D/Q can be increased. Hence, overload can be further mitigated. Exemplary thereof is one known arrangement where the bit rate is reduced by signaling the activity status once each multiframe where a multiframe includes a plurality of frames. Thus, by signaling the activity status of each trunk only once during, for example, 24 frames, rather than transmitting the status bits for all trunks during each of the 24 frames, still further bandwidth reduction is obtained. Notwithstanding, known DSI arrangements for mitigating overload by data compression typically introduce excessive quantization noise.
Accordingly, a broad object of our invention is to provide an improved digital communications system for mitigating overload and its unfortuitous effects.